Silicium aus der Gasphase : Optimierung der Gasphasenabscheidung von Silicium aus Silan

Silicon is an important material with a huge variety of technical applications. The manufacturing of silicon films via chemical vapor deposition (CVD) is widely used in the electronics and photovoltaic industry. While only thin films are required for electronics applications because of decreasing device size, rather thick films and therefore high growth rates are desired for solar cells to reduce costs per unit. Most investigations of silicon CVD with high growth rates concentrate on the use of chlorinated silanes as precursors which have been successfully applied in this respect. To reduce corrosion problems with the chlorinated chemicals and for ecological reasons, silane has come into the focus of interest. Therefore, the present study addresses the optimization of the deposition rate using silane precursor in silicon CVD. Furthermore, attempts have been made to overcome the major drawback of silane, its pronounced tendency to form particles in the gas phase during conventional CVD applications. In a first step, the gas phase pyrolysis of silane in different diluent gases has been analyzed with model calculations. Results of this model regarding the time scale of the gas phase reactions and the influence of silane in different buffer gases have been used in the definition of suitable deposition conditions for the experiments. In the model, intermediate species in the silane pyrolysis are considered, growth species being mainly silylene, SiH2, and disilane, Si2H6. Since Si3 is the largest silicon species in the reaction mechanism, its concentration is used as an indicator for gas phase nucleation. Model calculations have considered silicon deposition on a substrate and resulted in simulated flow profiles of species concentrations and growth rates. At low speeds of the gas flow, high concentrations of the growth species SiH2 and Si2H6 as well as of the undesired particle-forming species like Si3 are predicted. At higher velocities, the concentrations of the particle-forming species decrease more rapidly than those of the growth species giving indications about an optimal range of flow velocities for the deposition. The dependence of the growth rate on silane concentration shows that growth rates and the tendency to particle formation both increase with increasing silane content of the gas mixture. The growth rate also increases with substrate temperature. Inert buffer gases show a maximum deposition rate at about 1200 K while with hydrogen as the buffer gas the growth rate increases steadily but no maximum is observed. The results of the experiments carried out with 2 - 18 p.c. silane at 1000 - 1210 K are in good agreement with the model calculations. The highest growth rate observed is about 2 µm/min for nitrogen and 5 - 6 µm/min for hydrogen or helium as diluent. With respect to previous investigations using silane as a precursor in thermal CVD, this is an improvement by about one order of magnitude. Surface roughness of the deposits is higher with helium or hydrogen as diluent, which can mostly be accredited to the slower growth in nitrogen. The apparent activation energy of the deposition process is in the range of 116 - 160 kJ/mol which agrees well with results from previous work by other groups. The deposited films consist mainly of polycrystalline silicon which has been demonstrated by XRD-analysis and Raman spectroscopy.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bielefeld

Year

2002

Author dc:creator

• Hofstätter, Michael

Repository record source_url

https://pub.uni-bielefeld.de/record/2305455

OAI identifier oai:identifier

oai:pub.uni-bielefeld.de:2305455